This invention relates to the display of three dimensional (3D) video images on a video display. The term video is to be taken as the display of successive frames of still images collectively capable of providing a moving image.
Most moving displays are two dimensional, for example television (video) and movie films. For many applications movement of two dimensional images is adequate because an observer can obtain stereoscopic information from movement.
In non-moving (still) displays two dimensional displays are most common and are quite cheap to produce, by e.g. photocopying, printing, photography. Two dimensional images are provided by displays whose amplitude (amount of darkness and or colour) varies with position within the image. True three dimensional still displays can be provided by holographic techniques with photographic plates for example. Other 3D effects can be provided by a technique known as autostereo.
A three dimensional image can be provided in holographic films by variation of amplitude or phase within the hologram. An observer sees a slightly different image by each eye and at different positions (distance and angular) from the display. A big difference between the two types of display is the amount of information needed, with at least an order of magnitude more data needed for the 3-D case.
Holographic images have been displayed using liquid crystal devices, but suffer from very narrow fields of view, i.e. the stereoscopic effect is only observed over a narrow range of angles.
Moving video displays are formed by displaying frames of still images at a high enough rate that an observer can not detect individual frames, and instead sees a seamless movement. To display 3-D images with movement requires data rates much higher than is easily and cheaply available, especially for large displays.
According to one aspect of the invention, the problem of angle of view is solved by a three dimensional display which includes a spatial light modulator (SLM) on which a repeated sequence of patterns is illuminated by collimated light that scans repeatedly through a series of discrete directions in synchronisation with the holograms.
The term xe2x80x9cpatternxe2x80x9d is used in this specification to mean a two dimensional variation in phase or amplitude which could comprise the hologram, autostereo distribution or image to be projected.
According to this invention, a three dimensional video display includes: a screen on which successive frames of patterns can be displayed and means for projecting an array of collimated light beams which can each illuminate the whole screen through a series of discrete directions in synchronism with the display of successive frames;
wherein said screen includes a layer of liquid crystal material located between a front plate and a rear plate; a layer of light sensitive material, operable to change the voltage on the adjacent layer of liquid crystal material; a reflective layer between the liquid crystal material and the light sensitive material and electrode structures on the inner faces of plates for applying a voltage across the liquid material, the electrodes forming collectively an array of separately addressable segments in the liquid crystal layer and each collimated light beam is arranged to illuminate the whole of front plate.
The screen may comprise a plurality of separate areas, each independently addressable in field time to form collectively a complete frame of a pattern: for example, the screen could be formed by an optically addressed spatial light modulator having a plurality of separately addressable segments each arranged to receive a sub-pattern in a field time.
In a preferred embodiment, means for projecting patterns on the image is included.
The means for projecting may include means for projecting a plurality of field patterns in sequence on spatial separate areas (A to P) of the image screen or light sensitive layer to form a complete frame of an image and may further include a digital micro mirror device (DMD) having a matrix of separately addressable pixels collectively providing a pattern or part of a pattern for projection on to the screen.
The means for projecting could include a cathode ray tube video display for projecting an image or a part of an image on to the screen.
The invention may include an optical arrangement wherein the means for projecting an array of collimated light beams is arranged substantially in the focal plane of the optical arrangement.
The display of claim 1 where each collimated light beams may be laser light and may be of different wavelengths.
A grating may be included between the projecting means and the screen.
Ideally the images on a video display should be three dimensional like those recorded by holograms so that changes in parallax can be seen as the observer moves closer to or further from the display. and so that the observer can gauge depth by inspecting the image from different viewpoints and by using binocular vision. Holograms are merely high resolution displays and it is possible to display a video hologram on a liquid crystal display, However liquid crystal layers tend to be at least 1.5 microns thick, so the pixels on a liquid crystal tend to be no smaller than 2 or 3 microns, and it follows that the field of view of a hologram on a liquid crystal display is little more than a few degrees. Furthermore the cost of liquid crystal displays scales with resolution in such a way as to make the display of video holograms uneconomic.
Three dimensional images can also be displayed using autostereoscopic pixellation, where the screen comprises a two dimensional array of pixels each of which rather than being Lambertian (as for the display of two dimensional images) controls the intensity of light as a function of ray direction. A simple way of making an autostereoscopic display is to illuminate a liquid crystal display with a continuously scanning spot source of light in the focal plane of a lens. The lens and light source produce rays all travelling in one general direction at any one instant, and if the direction of the rays is synchronised with the display of appropriate views of a solid object on the liquid crystal display, then the eye will integrate a three dimensional image over time. The problem here is that in order to avoid the observer seeing flicker, the liquid crystal display must switch quickly and for good quality three dimensional images this is once again uneconomic.
Unless the positions of all observers are known then three dimensional images almost inevitably require data rates to be an order of magnitude greater than for two dimensional images. Any design of three dimensional display should provide a way of handling these data, but the higher the data rates, the less capacitance can be tolerated on the transmission lines which carry data onto the screen, and therefore the smaller the screen.
Small displays with high frame rates can indeed be made cheaply, for example by lacing a layer of ferroelectric liquid crystal on a silicon substrate, or with a digital micromirror device or cathode ray tube. But users want large three dimensional images, and even with devices where a three dimensional image can be displayed, the effect of magnifying the image to a usable size is to make the field of view too narrow.
Transmission line effects become less relevant if data is transferred in parallel, and this is what happens on light valves (or optically addressed liquid crystal displays) which comprise a sandwich of a light sensitive layer and a light modulating layer such as liquid crystal. When a voltage is applied across the light sensitive layer and the light modulating layer together than an image projected onto the light sensitive layer is transferred across to the light modulating layer, and amorphous silicon/ferroelectric liquid crystal light valves have frame rates of several kilohertz. Furthermore the layers in a light valve are in principle unpatterned, so one can get a device which acts as a high resolution, high frame rate liquid crystal display without the cost of high resolution lithography. If images from a small display are projected onto the back of a light valve, it is possible to assemble a device capable of projecting high resolution two dimensional images from relatively cheap components and various configurations have been proposed. The light valve can be larger than the liquid crystal display so can have higher resolution, but the liquid crystal display can nevertheless address all parts of the light valve if the image of the liquid crystal display is multiplexed across the light valve so that it addresses adjacent areas of the light valve one by one within each light valve frame. The magnification associated with projection makes these configurations inappropriate for the display of three dimensional images, but because they are unpatterned, light valves can in principle be made as large as television screens and still be relatively cheap.